Speculations on the Origin and Evolution of
Continental Crust
• Earth’s thermal evolution poorly understood
- parameterized models yield contrasting predictions w.r.t. onset of plate tectonics
- possibility of discontinuous transitions
• Models of continental growth are widely disparate due to:
- Differing views of continental age distribution as growth or preservation record
- Differing views of origin and evolution of plate tectonics
- Changing estimates of relative arc magmatism vs. subduction erosion rates
- Differing lessons taken from other terrestrial planets
- Differing views on importance of ‘freeboard’
- Differing emphasis & views of trace elements & isotopes
- Continental composition reflects growth model and v.v.
- Untested assumptions regarding crustal composition
?
• Composition of the continental crust
- Diverse compositional estimates, particularly regarding nature of the lower crust
- Disagreements about the composition of arcs
Heat Sources & Sinks
dT/dt  Heat Sources (H) - Heat Loss (Q)
Heat Source/Sink
Heat Production
melt extraction
Total Heat Loss (Q)
convection
conduction
Tm
Temperature
Discontinuous transitions
Heat Source/Sink
melt extraction
plate tectonic
convection
stagnant-lid
convection
conduction
Tm
Temperature
Discontinuous transitions
melt extraction
Heat Source/Sink
Heat Production
plate tectonic
convection
stagnant-lid
convection
conduction
Tm
Temperature
Discontinuous transitions
melt extraction
Heat Source/Sink
Heat Production
plate tectonic
convection
stagnant-lid
convection
conduction
Tm
Temperature
Discontinuous transitions
melt extraction
Heat Source/Sink
Heat Production
plate tectonic
convection
stagnant-lid
convection
conduction
Tm
Temperature
Discontinuous transitions
melt extraction
Heat Source/Sink
Heat Production
plate tectonic
convection
stagnant-lid
convection
conduction
Tm
Temperature
Discontinuous transitions
melt extraction
Heat Source/Sink
Heat Production
plate tectonic
convection
stagnant-lid
convection
conduction
Tm
Temperature
Discontinuous transitions
melt extraction
transition
Heat Source/Sink
Heat Production
plate tectonic
convection
stagnant-lid
convection
conduction
Tm
Temperature
Discontinuous transitions
melt extraction
Heat Source/Sink
Heat Production
plate tectonic
convection
stagnant-lid
convection
conduction
Tm
Temperature
Discontinuous transitions
melt extraction
Heat Source/Sink
Heat Production
plate tectonic
convection
stagnant-lid
convection
conduction
Tm
Temperature
Discontinuous transitions
melt extraction
Heat Source/Sink
Heat Production
plate tectonic
convection
transition
stagnant-lid
convection
conduction
Tm
Temperature
Do such discontinuous transitions occur?
Sleep 2000
• Uhh…maybe
Continental Crust Growth Models
Harrison (2009)
Fyfe (1978): Early Continents with
Greater Continental Mass at ~2.5 Ga
• Lots of early continental crust
• Unique model: present crustal
volume not peak value
• Major role for ancient hotspot
addition to continental crust + plate
boundary interactions
• Evidence:
– Subduction mass balance
indicates shrinking
– Higher freeboard in the past
may indicate more continent
Armstrong (1981): Steady State Recycling
• All terrestrial bodies differentiated at 4.5
Ga into constant mass core, depleted
mantle, enriched crust & fluid reservoirs
• Steady state crustal mass achieved by
early Archean.
• Evidence:
- Uniform thickness of CC with age
- Constancy of freeboard
- Arc magmatism & sediment
subduction currently about equal
- Mantle Sr & Nd isotopes consistent
w/ recycling constant continent mass
- Recycling model fit growth estimate
of Hurley & Rand (1969)
Warren (1989): Present Volume by ~4 Ga
•
Similar to Armstrong (1981) but
with near steady state achieved
even earlier.
•
Based on an analogy to the growth
history of the lunar crust.
•
The initial continental crust is
anorthositic to tonalitic but
comparable buoyancy to present
day
Reymer & Schubert (1984): Early Continents
Followed by Slow Growth
•
•
•
•
Based on Phanerozoic island arc
growth rates (note: all arc material
assumed primary)
Includes Archean growth rates 3-4
times the present rate
Also considered: hot spot
contributions to the crust.
Evidence:
– island arc mass balance (&
scaling by heat production)
– Constant freeboard actually
requires growth due to
deepening ocean basins w/ time
Brown (1979): Minor Hadean Continental
Crust Followed by Slow Growth
•
•
Minor early continental crust with
slow growth since Early Archean
The evidence:
– Brown disputes significant
sediment subduction
– Modern accretion rates fit a
growth model if corrected for
higher heat flows with age
– Granites predominately reflect
mantle addition, so  higher
crustal addition rates
Campbell (2003): Minor Hadean
Continental Crust with Slow Growth
•
Similar to Brown’s model in the
rates and timing of growth.
• But even less crust in the early
Hadean
• Evidence
- Nb-U-Th systematics in mantle
derived from 2.7-3.5 Ga volcanics
O’Nions et al. (1979): Slow Continental
Growth Since ~2.5 Ga
•
Two-reservoir box model w/ timedependent coefficients for transport
between the reservoirs
•
Generation of continents involved > half
of mantle
•
Maximum rate of continental growth
between 3.5-2.5 Ga (present day rate
only 20% of max)
Dewey & Windley (1981): Slow Continental
Growth Since ~2.5 Ga
•
Emphasis on decline in heat production
from smaller, thinner, faster moving
plates to slower, thicker, slower moving
plates 1/6th the Archean rate:
– 85% of CC by 2.5 Ga
•
Based on early Proterozoic indicators that
plate interacting w/ a lithosphere of
similar size to present:
– Large continental areas show high
degree of structural cohesion
– Widespread basement reactivation
adjacent to linear thrust belts (i.e.,
like present)
•
Also: lots of high-K minimum-melting
granites over calc-alkaline rocks at 2500700 Ma implies dominance of crustal
differentiation over growth
Allègre (1982): Slow Continental Growth
Since ~2.5 Ga
• Box modeling of Nd-Sr correlation
interpreted due to rapid growth of
continental crust at ~2.5 Ga
• Sr-Nd isotope systematics viewed as
evidence of ‘continental pumping’
• Mean age of continents of 2.5 Ga
continents were formed throughout
geological time and not suddenly
• Assumes knowledge of mantle
volume depleted by crust formation
and composition of undepleted
mantle
McLennan & Taylor (1982): Slow
Continental Growth Since ~2.5 Ga
•
No significant change in REE and Th
abundances in post-Archean shales
•
Modeling of REE and Th abundances suggest
minimum ratio of post-Archean to Archean
upper CC required to eliminate Archean
upper crustal signature is ~4:1
•
They propose 65-75% of CC formed during
3.2-2.5 Ga and 70-85% formed by 2.5 Ga –
consistent w/ continental freeboard over
past 2.5 Ga
Collerson & Kamber (1999): Slow
Continental Growth Since ~2.5 Ga
•
•
•
•
•
Th, U, and Nb are strongly incompatible
elements during the melting of mantle
Differences in CC, undifferentiated mantle,
and depleted mantle:
– A deficit of Nb in relation to Th & U
Thus differences in U & Th vs U can be used
to infer crustal mass through time
Recycling of CC is most likely reason for
decoupling U and Th due to soluble U in
oxygenating atmosphere
Strong net growth recorded between 3.02.0 Ga, slowed down after 2.0 Ga due to
increased erosion, and renewed increase of
growth from ~250 Ma to present day
shows faster growth during times of
continental dispersal
Veizer & Jansen (1979): Slow Continental
Growth Since ~2.5 Ga
•
Basement and sedimentary recycling
•
Measured cumulative age distribution:
– continental age provinces
– areas and thicknesses of seds
– mineral reserves
•
Distributions follow an exponentially
increasing function due to recycling
•
Simulation favors continual CC growth
through time w/ slow growth in early
Archean & fast at 3.0-2.0 Ga
•
Sediment chemical & isotopic trends
support a mafic  felsic transition in the
CC at ca. 2.5 Ga
•
Sm/Nd suggests sedimentary cycle is
~65% cannibalistic system, thus present
day sedimentary mass is more mafic than
upper CC"
Hurley & Rand (1969): Linear Growth of
Continents Since ~3.8 Ga
•
•
•
K-Ar ages of continental crust:
– All available age data
representing ~2/3 of
continental area
– Age patterns represent mix of
primary ages and thermal
overprint
– Growth of continents largely
peripheral and concentric
about Laurasia & Gondwana in
pre-drift positions
Histogram of areal extent of crust
shows accelerating generation
starting at 3.8 Ga
Problem: K-Ar ages unlikely to
record continental growth
Hacker et al. (2011): Continental Relamination
During subduction, mafic rocks
become eclogite & sink whereas
SiO2-rich rocks are transformed
into less dense felsic gneisses
These felsic rocks may rise
buoyantly, undergo
decompression melting &
relaminate at base of the crust
Thus the lower crust need not be
mafic & the bulk continental
crust may be more SiO2
enriched than typically thought
Preservation vs. Growth: Age Provinces
Sm-Nd model ages
of basement rocks
from Australia,
North America
and Scandinavia
Bennett & McCulloch (1994)
If this is growth record, why does heat production vary systematically with
age province?
Are we confident that our sampling distribution is adequate?
Preservation vs. Growth: Detrital Zircon
- 8 peaks on 5 more cratons @ 0.75, 0.85, 1.76, 1.87, 2.1, 2.65, 2.7 & 2.93 Ga
reflect subduction system episodicity but not on continental/supercontinental scale
- 5 major peaks at 2.7, 1.87, 1.0, 0.6 & 0.3 Ga closely tied to supercontinents
Condie & Aster (2010)
Does Continental Crust Form in Arcs?
Widespread view that composition of arcs ≠ continental crust
CaO
15
10
5
0
Courtesy Jon Davidson
5
10
15
MgO
20
Primary arc magma ≠ continental crust
Explanations:
• We’ve misestimated the composition of the
continental crust
• We’ve misestimated bulk arc composition
• Primary arc magmas are not high MgO (could be
slab melts?)
• Crust formed in the past by a different mechanism
• There is a complementary crust-mantle return flux
of cumulates/residues
Delamination of mafic
cumulate
removal of ultramafic cumulate by
delamination through density instability
following orogenesis
differentiate
seismological
Moho
genetic
Moho
cumulate
removal of ultramafic
cumulate through thermal
erosion associated with
wedge convection
magma input from sub lithosphere
= primitive arc magma
Courtesy Jon Davidson
Longstanding assumptions of
regarding continental crust
1) The crust is vertically stratified
from mafic to felsic
Ingebritsen and Manning (2002)
“(metapelites) have velocities that overlap the complete velocity range displayed by
meta-igneous lithologies” (Rudnick and Fountain, 1995)
2) U, Th, K are redistributed upward to create a thin radioactive layer
- geophysical basis of observation non-unique
- proposed mechanisms for upward transport in the crust not viable (e.g., anatexis
enriches lower crust in U and Th; high aCO2) or untested (e.g., brines)
- granulites not clearly depleted in U, Th & K
- estimates of heat generation of lower crust differ by factor of two
3) Orogenesis is a bit player in establishing crustal architecture
“(Orogenic P-T paths) are probably not representative of the deep crust but are merely
upper crustal rocks that have been through an orogenic cycle” (Rudnick and Fountain,
1995)
K, U and Th in granulites typical of ‘average continental crust’
(Rudnick et al., 1985)
Can tectonic models
tell us about crustal
structure & mass
transfer?
Continental crust is portion of Earth
furthest from thermodynamic equilibrium
>90% processed through 1 orogenic cycle
- Is this circular (e.g., assumes distribution of radioactivity)?
- Is there a process whereby a homogenized crust returns rapidly to a
stratified state?
- Are models sufficiently well-constrained; i.e., do free parameters overwhelm
constraints?
- Seismic cross sections & active orogens appear inconsistent with
assumption that surface rocks characterize the crustal column
Numerous tectonic models; most emphasize horizontal transport
1920's
S
1990's
N
ITS Tibetan Plateau
ITS
Underthrusting of India
(Argand, 1924)
Steady-state accretion/erosion
(Royden, 1993)
?
ITS
1970's
ITS
Underthrusting of Asia
(Willett and Beaumont, 1994)
Distributed shortening
(Dewey and Burke, 1973)
STDS
MFT MBT
RZT
MCT
HHL
MCT
Extrusion of Tibetan mid-crust
(Nelson et al., 1996)
NHG
Hot iron model
(LeFort, 1975)
ky
partially molten
100 km
no vertical exageration
MHT
H2O + CO2
1980's
ITS
ITS
Plateau inheritance
(Murphy et al., 1997)
Convective mantle lithosphere removal
(Houseman et al., 1981)
MCT Zone
MBT
Out-of-sequence thrusting
(Harrison et al., 1998)
Siwalik
Gp.
MHT
ITS
Hydraulic injection
(Zhao and Morgan, 1985)
2000's
ITS
Channel flow/extrusion
(Beaumont et al., 2001)
ITS
?
Delayed underplating of India
(Powell, 1986)
ITS
Oblique stepwise growth
(Tapponnier et al., 2002)
Why such disparate continental growth rates?
• Differing views whether present continental age distribution is a growth or
preservation record?
• Differing views of origin and evolution of plate tectonics
• Changing estimates of relative rates of arc magmatism and subduction erosion
(0.1-1 km3/yr in 80s; currently ~3-5 km3/yr for both)
• Differing views on lessons from other terrestrial planets
• Differing views on importance of freeboard arguments
• Differing emphasis & views of trace elements & isotopes
• Knowledge of the composition of the lower continental crust is poor
• Estimates of the composition of the continental crusts reflects how the estimator
think it forms and grows and v.v.
When Did Plate Tectonics Begin?
Stern – Chinese Bull. Sci. 2007
Preserving Original Structures in Multiply Deformed
Old Rocks – Not Easy!
Nuvvuagittuq, Quebec
Melting in a Convergent Margin Involves Fluids Released from the
Subducted Slab These are characterized by incompatible element
enrichment, particularly Pb, but also Nb, Ti depletion.
Stern, RoG 2002
The “Granitic” component of Archean crust
TTG – Tonalite, Trondhjemite, Granodiorite
Martin et al., Lithos 2005
High-Ti
15
High-Ti
10
y
c
n
ue
q
e
rF
5
0
15
Depleted Low-Ti
Basalt
Basaltic
andesite
Andesite
depleted Low-Ti
10
y
c
n
ue
q
e
rF
5
0
15
Enriched Low-Ti
enriched Low-Ti
y
c
n
ue
q
e
rF
10
5
0
42 44 46 48 50 52 54 56 58 60 62 64 66 68
SiO2 (wt. %)
Nuvvuagittuq Mafic Crust
Arc tholeiites and boninites at 4.4 Ga?
O’Neil et al., J. Pet. 2011
Another Consequence of Subduction:
Injecting Crustal Material into the Mantle
Preservation of Eclogitic Diamond
Diamond
inclusion sulfide
sulfur isotopic
composition
Blue Triangles
Archean Sediments
Shirey and Richardson, Science 2011
Farquhar et al., Science 2002
Green Diamonds
Post-Archean Sediments
Eclogites in the Mantle
The Start of Subduction, or the Start of Preservation?
-1
SAF2000P
B
-0.5
0
0.5
1
P-wave velocity anomaly (%)
B
0
100
300
400
500
600
700
800
900
1000
B: ( 34.25S, 19.25E )
Carlson et al., RoG, 2005
B : ( 18.50S, 31.50E )
Depth (
km)
200
Re-Os model ages for many peridotite
xenoliths from the subcontinental
lithospheric mantle provide age peaks
near 2.9 Ga. Mantle lithosphere cool
enough and thick enough to retain the
evidence of subduction?
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Pearson &Wittig, ToG, in press
Diamond Inclusions from the Panda (Slave Craton) Kimberlite:
A 3.5 Ga Re-Os age and a high initial 187Os/188Os suggestive of formation from a crustal
component with high Re/Os
Diamond Inclusion Age
3.52 ± 0.17 Ga
gOs = +6
Panda (Slave Craton, Canada) diamond inclusions and harzburgite
xenoliths (Westerlund et al., CMP, 2006)
Why such disparate continental growth rates?
• Differing views whether present continental age distribution is a growth or
preservation record?
• Differing views of origin and evolution of plate tectonics
• Changing estimates of relative rates of arc magmatism and subduction erosion
(0.1-1 km3/yr in 80s; currently ~3-5 km3/yr for both)
• Differing views on lessons from other terrestrial planets
• Differing views on importance of freeboard arguments
• Differing emphasis & views of trace elements & isotopes
• Knowledge of the composition of the lower continental crust is poor
• Estimates of the composition of the continental crusts reflects how the estimator
think it forms and grows and v.v.